High temperature electrically-heated furnace



March 25, 1947. s. HQSTUPAKOFF HIGH TEMPERATURE ELECTRICALLY-HEATEDFURNACE Filed June 2, 1944 rif.-

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Patented Mar.l25, 1947 HIGH TEMPERATURE ELECTRICALLY- HEATED FURNACESemon H. Stupakoi, Latrobe, Pa., assigner to Stupakof! CeramicManufacturing Co., Latrobe, Pa., a corporation of PennsylvaniaApplication June 2, 1944, Serial No. 538,393

2 Claims. 1 A

This invention pertains to electrically-heated furnaces, and moreparticularly to electricallyheated furnaces using resistance heaters,and is for a furnace 'capable of operating at high temperatures in anoxidizing atmosphere.

According to the present invention, it is proposed to use as the heatingelements for the furnace, a ceramic material which is a conductor of thesecond class. Material available commercially under the trade nameInsulcon is a typically satisfactory material. Such materials are madefrom a mixture of rare earth oxides and have a negative coefilcient ofelectrical resistance. They are insulators when cold, but when-heated toabout 700 C. or above, they will readily conduct electricity and thepassage of current therethrough will of course thereafter sustain themat a conducting temperature.

The melting temperature of the conductor will lvary according to theingredients used, but may be in the neighborhood of 2700 C. Beingcornposed of completely oxidized materials, a conductor of this kind ischemically' stable when heated in an oxidizing atmosphere up to or nearits melting temperature.

For many years, I have tried to use Insulcon" conductors as a heatingmeans for furnaces intended to operate with anoxidlzing atmosphere attemperatures at or near, or possibly somewhat in excess of 2000 C.,there being an industrial need for a furnace of this character. However,difficulty has been encountered in providing for the conduction ofelectrical current to the heaters operating at or above this region oftemperature. Platinum is the metal most resistant to oxidization at hightemperatures, but platinum has a melting point of approximately 1650"C., so that the temperature of a furnace using these oxide heatingelements, where platinum is used to carry current to the elements, isconfined to a temperature below 1650 C. Metals which melt at a highertemperature than platinum, suchas molybdenum (melting point 2620 C.), ortungsten (melting point 3370 C.), oxidize or burn readily when heated inair or other oxidizing atmospheres, so

that such metals are of no value as leads or terminals for "Insulcon orsimilar heaters.

According to the present invention, I have found that by using highfrequency electrical curent, perhaps 100,000'jf2cycles, more or less,electrical current can beaconducted from a lead for a reasonabledistance` through air to the "Insulcon units. This avoids the necessityfor Iany metal terminals on the Insulcon units themselves. and henceenables `the furnace to be oper- `a furnace embodying my invention;

Fig. 2`is a transverse section in the plane of line II-II of Fig. 1;

Fig. 3 is a fragmentary view illustrating partly in section and partlyin elevation, an arrangement for including the ceramic heaters in a highfrequency circuit; and

Fig. 4 is a detail section in the plane of line IV--IV of Fig. 1,showing the relation oi the Insulcon or other heaters to the source ofcurrent supply.

In the drawings,4 the structure has been illustrated only schematicallyfor the purpose of indicating one manner of employing my invention. Inthe drawings, 2 designates generally a. refractory furnace structurewhich is here illustrated to be a small capacity or laboratory size ofunit. It has a refractory bottom 3, refractory side walls I, and arefractory top 5, enclosing a chamber C. In the roof of the furnace aretransversely extending hangers or inverted piers 5a, also formed oi'refractory material, and having a plurality of small holes extendingtransversely therethrough. These holes loosely receive the heatingelements or conductors 6. Provision is made for several of theseconductors to be supported in parallel relation.

As heretofore indicated, the conductors or elements 6 are conductors ofthe second class formed of Insulcon or a like compound of rare earthoxides having a negative coeii'lcient of electrical resistivity andwhich are non-conducting when cold, but which become conductors whenheated to a temperature at or above 700 C.

The ends of the conductors 6 terminate in spaced relation towater-cooled electrodes l and 8, one of these electrodes being at oneend oi the resistors and one being at the opposite end, the electrodesbeing spaced from the ends of the resistors in such manner that areasonably short gap exists between the resistors and the electrodes.The electrodes themselves are illustrated as being formed of metal witha water-circulating space 9 therein. Each electrode has a connection fortwo water pipes, these being designated i0, so that water may becontinuously circulated therethrough, enabling the electrodes to be keptcool enough to prevent damage thereto. In the drawings, the terminals 1and 8 are shown as being located entirely within the furnace chamber,but it is understood that provision may` be made for locating themotherwise, or surrounding them with heat insulating material to avoidexcess loss of furnace heat. The water pipes I may also convenientlycomprise conducting leads whereby the electrodes may be coupled into ahigh frequency circuit includingthe wires I I and a source of highfrequency current I2. By high irequency I mean a frequency above therange of audibility and preferably of the order of around 100,000 cyclesor more. Any frequency that will f able the required amount of currentto trans- 'se the air gap between the electrodes and the :,r the ceramicconductors 6 may be employed. While frequencies considerably in excess100,000 cycles may be used, there is little adultra-high frequencies.135:. f z necessary for the rods 9 to be "o a conducting temperaturebefore viii operate electrically, This may use ci' gas burners or anyother y means not shown, which can be employ to bring the furnace up toa, temperature of around 000 C. and thereafter removed or the operationthereof discontinued. At this time the high frequency circuit will beenergized and our will travel. from the electrodes through the airrgapat each end thereof .into or from the conductors 6, which, according tothe amount oi current employed, can be operated in an atmosphare of airor other oxidizing atmosphere at a` temperature of about 2000 C. orgreater, the ltempera'ture however v being maintained somewhere at apoint below themelting point of the conductors around 270W C.

The heating units. themselves are preferably rod-like in form 'and ofrelatively small cross section. This section may be, for example, of theorder of a quarter of an inch or less. Where the cross section is large,there may be n0n-unlform temperature lconditions within the unit itselfdue to the tendency oi the current to seek a path through the conductorcausing overheating in Some polntsrand perhaps underheating in others.The smaller the diameter of the rods, the more nearly the likelihood ofthere being uniform conductivity across the entire section, and lesslikelihood of the current seeking a path of less section than the fulldiameter of the sectionD It will of course he understood that thedrawing is schematic and that the heating units may be arranged at thebottom or sides of the furnace chamberu While I have illustrated anddescribed one embodiment or my invention and the method of operating thesame, it will be understood that this merely we? of illustration andthat various changes and modifications may be made within thecontemplation of my invention and under the scope of the `followingclaims:

i claim, as my invention: y

l., electric furnace designed to operate at very high temperatures in anoxidizing atmos 0 Number phere, comprising an enclosure, arod-likeheating element supported therein comprising rare earth oxides andhaving a negative coemcient o! electrlcalresistivlty, a pair ofterminals o! high electrical conductivity each being in proximity to butspaced from diil'erent ends of said conductor, means for cooling saidterminals, and a high frequency circuit connected across said terminalst0 pass a high frequency current throughout substantiallLthe entirecross-sectional area of said heating element.

2. An electric furnace designed to operate at very high temperatures inan oxidizing atmosphere, comprising furnace enclosing walls. a pluralityof rod-like conductors of the second class supported from a portion o!said walls, a pair of hollow terminals of high electrical conductivity,each being in proximity to but spaced from corresponding ends of saidplurality of conductors of the s econd class, means for circulating ac001- ing fluid through said hollow terminals so as to maintain theirtemperature substantially lower than that of said second classconductors, and a source o1 high frequency current connected in serieswith said terminals for passing a high trequency current throughoutsubstantially the entire cross-sectional area ct said rod-likecondoctors of the second class for maintaining them at very hightemperatures after they have been brought up to such temperatures so asto be# come conducting. v

SEMON H. S'IUPAKOFF.

REFERENCES CITED The following references are oi record in the ille oithis patent:

UNITED STATES PATENTS Name Date Shaw Apr. 12, 1932 Saunders Sept. '7,1920 Dowiatt Oct. 29, 1929 Murthrup Mar. 3, 1931 .Benner et al July 19.1932 Fowler Nov. 21, 1882 Colby May 13, 1924 Hull Jan. 6, 1925 AdamsJune 24, 1930 McArthur Mar. 7, 1933 Meissner Apr. 10, 1934 Rouy Dec. l5,1942 Millar Nov. 24, 1931 Ruckstahl .1 May 10, 1932 Benner et al Apr.16, 1935 Henke Dec. 13, 1938 Geller Aug. 22, 1944 FOREIGN PATENTSCountry Date Australian Apr. 10, 1929 French June 11, 1934 German Oct.13, 1939 British Dec. ll, 1912 Number

